1. Field of the Invention
This invention relates generally to expandable intraluminal vascular grafts, generally referred to as stents, and more particularly concerns biodegradable mesh stents reinforced with a biodegradable film laminate capable of releasing therapeutic drugs.
2. Description of Related Art
Stents used to maintain patency of vessels in the body are typically implanted within a vessel in a contracted state, and expanded once in place in the vessel to allow fluid flow through the vessel and the stent. Such a stent can be moved along a guide wire previously placed in the vessel, and expanded by inflation of a balloon within the stent. Deflation of the balloon and removal of the guide wire leaves the stent in place in the vessel, locked in an expanded state. It has been found that continued exposure of the stent to blood can lead to undesirable thrombus formation, and that continued existence of a stent in a blood vessel can lead to weakening of the blood vessel wall, possibly leading to arterial rupture or the formation of aneurisms. The stent can also become covered by endothelial tissue after implantation of the stent, after which the usefulness of the stent may be substantially diminished, and the continued presence of the stent may cause a variety of problems or complications.
It is therefore desirable that the material of the stent be biodegradable, or bioabsorbable, to reduce the risk of thrombosis or injury to the blood vessel. It is also desirable that the stent be formed of material of minimal thickness, so as to minimize blood flow blockage and facilitate biodegradability and bioabsorption. However, the material must have sufficient radial strength to function as a stent, so that it is desirable to reinforce the material, preferably also with biodegradable, bioabsorbable material.
It is also often useful to provide localized pharmacological treatment of a blood vessel at the location being treated with the stent. It would therefore be desirable to form a stent of materials that are capable of absorbing therapeutic drugs and releasing them at a predictable rate for an ascertainable period of time in a blood vessel, that are also biodegradable and bioabsorbable, and that can provide sufficient radial strength to maintain patency of a blood vessel.
Biodegradable fibers have been used in forming bone fixation plates. In using such fibers to form a biodegradable stent, the fibers can be formed as a mesh, which typically does not by itself have sufficient strength when fashioned as a stent to maintain patency of a blood vessel, and it is possible for the fibers to move around and separate. Chemically sizing the fibers to strengthen them can degrade the fibers, and solvent lamination of the fibers to strengthen them can leave an undesirable residue of solvent in the fibers. It would be desirable to strengthen the mesh of fibers and to prevent the fibers of the mesh from separating and moving around by laminating the mesh with a film of biodegradable, bioabsorbable material. However, in order to provide sufficient radial strength to the stent, it has been found that the density of fiber weave in the mesh should be closely packed, while such a close packing of fibers can prevent sufficient penetration of the film laminate into the fiber mesh to provide good bonding of a film to the fibers of the mesh, so that lamination of such a high density fibrous mesh is typically difficult to achieve, and inconsistent. It would be desirable to provide a stent made of a mesh of biodegradable fibers that can be closely packed for strength, and yet further reinforced with a biodegradable film lamination. The present invention meets these needs.